1. Field of the Invention
This invention relates generally to a method for rapidly heating a fuel cell stack in a fuel cell system and, more particularly, to a method for rapidly heating a fuel cell stack in a fuel cell system at system start-up that includes using a drive motor, a drive motor inverter or other power conversion device to generate waste heat to allow the fuel cell stack to have a high load.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs require certain conditions for effective operation, including proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
It is desirable during certain fuel cell stack operating conditions, such as fuel cell stack start-up, low power operation, low ambient temperature operation, etc., to provide supplemental heat to the fuel cell stack to provide the desired operating temperature, 60° C.-80° C., within the fuel cell stack for proper water management and reaction kinetics purposes. Particularly, the MEAs must have a proper relative humidity (RH) and the fuel cells must be within a certain temperature range to operate efficiently and produce the maximum output power.
At cold system start-up before the fuel cell stack has reached its desired operating temperature, the stack is generally unable to produce enough power to operate the vehicle. Therefore, the vehicle operator must wait a certain period of time until the fuel cell stack reaches its operating temperature as a result of stack inefficiencies before demanding significant load for operating the vehicle. Typical fuel cell stacks may take about 160 seconds or more to reach their operating temperature at which time they are able to provide power to operate the vehicle.
In a laboratory environment, heating the fuel cell stack can be accomplished by using a load bank to provide a controlled increase in load current to the stack, where the load current is limited based on the power available from the fuel cell stack. However, in the vehicle environment, road load is used to draw power from the fuel cell and cause it to heat up. However, the road load demand is not controlled by the fuel cell stack, but it is determined by the driver's demand. For example, if the driver starts the fuel cell engine and then drives at low speeds, the fuel cell stack will not warm up quickly and, as a result, the fuel cell stack will not be able to provide its full rated power if the driver suddenly demands it.
Existing methods to heat a fuel cell stack include the use of additional components, such as cell heaters or cooling fluid heaters and associated hardware for switching and controlling power to the heaters. However, the heaters and related hardware add cost, complexity and weight to the system, especially for heaters that would be large enough to achieve the desired stack operating temperature in the desired time. Also, hydrogen can be added to the cathode air or air can be added to the anode input to provide combustion and more quickly heat the stack. However, these techniques are limited because of the potential for degradation of the catalyst and its support structure, and the need to maintain safe gas compositions without the rapid release of excessive heat.